1. Field of the Invention
The present invention relates to a scanning probe microscope and a measurement method using the same, more particularly relates to a scanning probe microscope suitable for automatic measurement of side walls by measurement of fine relief shapes etc. on the surface of a sample like a wafer and a measurement method using the same.
2. Description of the Related Art
Scanning probe microscopes are known as measurement systems having measurement resolutions enabling observation of fine objects on the atomic level. In recent years, scanning probe microscopes have been applied to a variety of fields such as measurement of the fine relief shapes in the surfaces of wafers or substrates on which semiconductor devices are fabricated. There are various types of scanning probe microscopes for the different physical quantities for detection used for measurement. For example, there are scanning tunnel microscopes utilizing tunnel current, atomic force microscopes utilizing atomic force, magnetic microscopes utilizing magnetic force, etc. The ranges of their applications have been growing as well.
Atomic force microscopes are particularly suitable for detecting the fine relief shapes on sample surfaces and are proving their worth in the fields of semiconductor substrates, disks, etc. Recently, they have also been used in applications for in-line automatic inspection processes.
An atomic force microscope is basically configured provided with a measurement unit operating based on the principle of atomic force microscopes. The measurement unit is provided with a tripod-type or tube-type XYZ fine actuator formed utilizing piezoelectric devices. The bottom end of the XYZ fine actuator has a cantilever having a probe at its tip attached to it. The tip of the probe faces the surface of the sample. The cantilever is provided with, for example, an optical lever type photo detector. In the optical lever type photo detector, a laser beam emitted from a laser light source (laser oscillator) arranged above the cantilever is reflected at the back surface of the cantilever and detected by the photo detector. If the cantilever twists or bends, the position of incidence of the laser beam at the photo detector changes. Therefore, if the probe and cantilever displace, it is possible to detect the direction and amount of the displacement based on a detection signal output from the photo detector. An atomic force microscope is further provided with a comparator and controller as a control system. The comparator compares the detection voltage signal output from the photo detector and the reference voltage and outputs an error signal. The controller creates a control signal resulting in an error signal of zero and sends this control signal to the Z-fine actuator in the XYZ fine actuator. A feedback servo control system holding the distance between the sample and probe constant is formed in this way. It is possible to use this configuration to make the probe track and scan the fine reliefs on the sample surface and measure their shapes.
When the atomic force microscopes were first invented, the central issue was the use of their high resolution for measurement of fine shapes on the surface of dimensions on the nanometer (nm) order. At the present time, however, scanning probe microscopes have expanded in range of use to include in-line automatic inspection in the middle of in-line fabrication systems of semiconductor devices. In view of this, in actual inspection processes, it is required to measure the extremely sharp reliefs in the fine relief shapes on the surfaces of the semiconductor devices fabricated on wafers. At the present stage, automatic measurement of the surfaces of vertical walls or the side walls of holes having angles of 90 degrees, considered impossible in the past, is being sought for in-line inspection applications.
As technology for measuring vertical walls using atomic force microscopes, Japanese Patent Publication (A) No. 6-82248 and Japanese Patent Publication (A) No. 2001-249067 may be mentioned. Typical shapes of the cantilevers and probes disclosed in these publications are shown in FIG. 11. A probe 502 provided at the tip of a cantilever 501 is shaped similar to a cone at the area around the tip. That is, the probe 502 is formed by a straight part 502a and a cone-shaped tip 502b. FIG. 11 shows part of the area near the surface of the sample 503. The surface of the sample 503 is formed with grooves or holes having any depth, projections 504, etc. (hereinafter referred to as “grooves 504”). The probe 502 enters a groove 504 at the surface of the sample 503 and measures the shape of the groove 504. The probe 502 can be brought close to the surface parts of the side walls 504b and 504c of the groove 504 due to the shape of the cone-shaped tip 502b together with measurement of a bottom 504a of the groove 504. Therefore, by modifying the method of movement of the probe 502, it is possible to measure the side walls 504b and 504c of the groove 504.
As other technology for measurement of vertical side walls by the atomic force microscopes, there is the technology proposed by Japanese Patent Publication (A) No. 8-226926. An example of the state of measurement by the probe disclosed in this publication is shown in FIG. 12. The scanning probe microscope according to this publication is configured making a probe 512 at a tip of a cantilever 511 tilt (tilt method). FIG. 12 shows the state of tilt of the probe 512 and measurement of the surfaces of side walls 504b and 504c of the groove 504 formed at the surface of a sample 503. The probe 512 has a straight shape having a diameter of about 10 to 20 nm. Measurement is performed by making the probe 512 having this shape tilt by exactly a required angle (θ). A scanning probe microscope enabling measurement by this tilt method can be realized by a comparatively simple measurement system and can utilize currently technology to accurately measure the left and right side edges of projections etc., that is, side walls. By applying technology such as seen in recent carbon nanotubes to fabrication of probes, it is possible to realize the probe 512 of a diameter of about 10 to 20 nm. The practical applications are growing as well.
In the case of the system of the related art shown in FIG. 11, as circuit patterns of semiconductor devices (LSIs) fabricated on substrates become finer and the width a of the groove 504 becomes smaller than 100 nm—reaching the level of 50 nm in the future, the shape of the probe 502 having the cone-shaped tip 502b will make measurement impossible and the increasing fineness will not be able to be handled. In the case of this system, it would be necessary to make the diameter b of the cone-shaped tip 502b smaller than the width a of the groove 504 of the circuit pattern. For example, if making the diameter b of the cone-shaped tip 502b 40 nm, the diameter of the straight part 502a would become smaller than 10 nm. This is impractical from the viewpoints of fabrication and mechanical strength as well.
As opposed to this, in the system of the related art shown in FIG. 12, since it is possible to fabricate the probe 512 of a fine diameter and straight shape, it is possible to deal with the increased fineness of circuit patterns of semiconductor devices. In the case of the probe 512, the probe is formed by just a straight part. There is none of the problem with the diameter possessed by the cone-shaped probe and therefore it is possible to measure the side walls of the groove 504 even if semiconductor devices become finer.
On the other hand, with measurement by the tilt method shown in FIG. 12, the probe 512 is used for measurement by a single tilt posture, so there is the problem that it is only possible to measure one of the two side walls of the groove 504. To measure the two side walls of the groove 504, it becomes necessary to invert the tilt angle of the probe 512 and perform the measurement again one more time. Measurement systems of the atomic force microscopes configured to make the probe 512 having a straight shape tilt to the two sides to approach the side walls of the groove 504 and measure the shapes of the side walls have not yet exhibited sufficient technical advances at the present for the automatic measurement required for in-line measurement of semiconductor production lines. The reason why automatic measurement cannot be handled is that when changing the direction of tilt of the probe for measurement, at the time of the tilt, the positional relationship between the probe and sample changes before and after the tilt, so it is extremely difficult to measure the same location of a sample automatically from the two sides. No matter how high the precision the tilt and rotation mechanism provided, design of a practical measurement system is difficult in the field of measurement of fine dimensions of 50 to 100 nm. As explained above, in the field of automatic measurement, no atomic force microscope or other microscope practically realizing the tilt method has been realized.